Prior thermally insulated horizontal slider windows, having one fixed sash and one horizontally sliding sash, in which the frames are constructed with no direct metal heat-conducting path from the inner surface of a frame or sash member to the outer surface, if constructed to permit removal of the two sashes, had a small gap through which air could pass at the fixed sash meeting rail, to permit the fixed sash to be raised slightly during removal. To avoid drafts, occupants found it necessary to take special steps, each time the sashes were replaced in the frame, to seal this gap, as by stuffing material of a suitable nature into this gap.